The invention relates to a plastic injection-molding method for the production of a poly-V belt pulley and a belt pulley produced according to this method. This has an outer ring, an inner ring, and a ring web connecting the outer ring to the inner ring with a plurality of radially oriented ribs for supporting the outer ring against the inner ring. The injection molding is performed in a cylindrical injection-molding cavity whose poly-V ribbed inner shell is formed by a plurality of radially moveable slides and is ventilated during the injection molding via the gaps between the adjacent slides.
Such belt pulleys are typically used in belt drives of internal combustion engines for tensioning or guiding a poly-V belt (also called V-ribbed belt). The production is performed through plastic extrusion coating of the outer ring of a ball bearing that holds the belt pulley in a rotating manner on the internal combustion engine—see, for example, U.S. Pat. No. 8,617,016 B2. The removability of the poly-V ribbed running casing of the belt pulley from the injection-molding tool requires radially moveable slides that form, in the closed state, the correspondingly ribbed inner shell of the injection-molding cavity.
The ventilation of the cavity required for the extrusion coating is performed through the gaps between the slides. For the dimensioning of the gaps, however, the following target conflict results: a small gap causes the risk of air inclusions in the cavity, especially in the area of its axial end sides and circumferentially between the gaps and consequently of burn marks that reduce the tool service life. This risk increases with the width of the belt pulley and leads to unacceptable tool wear especially for PK5, PK6, and wider rib profiles. Increasing the gap promotes ventilation but negatively affects the roundness of the ribbed belt pulley running casing due to the high material accumulation on the gap with correspondingly impermissible burr formation.